Since liquid crystal display devices have merits such as high display quality, reduced thickness, reduced weight, and low power consumption, the use applications of the devices are expanding, and the devices are used for various use applications including mobile device monitors such as a mobile telephone monitor, digital still camera monitor, personal computer monitor, monitor intended for printing and design, medical monitor, and liquid crystal television. In association with the expansion of these use applications, it is demanded to further improve the image quality and the quality of the liquid crystal display device, and it is strongly demanded to improve luminance and to decrease power consumption by achieving higher transmittances specifically. Moreover, in association with the spread of the liquid crystal display device, a decrease in costs is also demanded.
In general, images are displayed on the liquid crystal display device in which an electric field is applied to the liquid crystal molecules of a liquid crystal layer sandwiched between a pair of substrates to change the alignment direction of the liquid crystal molecules and the change causes changes in the optical properties of the liquid crystal layer for displaying images. The alignment direction of the liquid crystal molecules when the electric field is not applied is defined by an alignment film that the surface of a polyimide thin film is rubbed. Conventionally, in an active matrix liquid crystal display device having a switching element such as a thin film transistor (TFT) for each pixel, an electrode is individually provided on a pair of substrates between which a liquid crystal layer is sandwiched, an electric field is set to a so-called vertical electric field that the direction of the electric field applied to the liquid crystal layer is almost perpendicular to the substrate surface, and images are displayed using the optical rotatory power of liquid crystal molecules forming the liquid crystal layer. For representative liquid crystal display devices in a vertical field mode, liquid crystal display devices in a twisted nematic (TN) mode and a vertical alignment (VA) mode are known.
In liquid crystal display devices in the TN mode and the VA mode, one of large problems is a narrow viewing angle. Therefore, as display modes to achieve wider viewing angles, an in-plane switching (IPS) mode and a fringe-field switching (FFS) mode, which is one type of the IPS mode, are known.
The IPS mode and the FFS mode are a so-called transverse electric field display mode in which an electrode is formed on one of a pair of substrates and an electric field to be generated has a component nearly in parallel with the substrate surface. Liquid crystal molecules forming a liquid crystal layer are rotated in a plane nearly in parallel with the substrate, and images are displayed using the birefringence of the liquid crystal layer. The IPS mode and the FFS mode are advantageous in that the viewing angle is wide and the load capacity is low as compared with the previously existing TN mode because of the in-plane switching of the liquid crystal molecules, for example. The liquid crystal display devices in the IPS mode and the FFS mode are regarded as new promising devices that replace liquid crystal display devices in the TN mode, and are in a rapid progress in these years.
In the liquid crystal display device, the orientation state of the liquid crystal molecules in the liquid crystal layer is controlled by the presence or absence of an electric field. In other words, upper and lower polarizers provided on the outer sides of the liquid crystal layer are set in the completely orthogonal state, a phase difference is generated due to the orientation state of the liquid crystal molecules between the polarizers, and light and dark states are formed. In order to control the orientation state in which no electric field is applied to the liquid crystal molecules, this control is achieved in which a polymer thin film called an alignment film is formed on the surface of the substrate and the liquid crystal molecules are arrayed in the array direction of polymers due to an intermolecular interaction caused by van der Waals force between a polymer chain and the liquid crystal molecule on the interface. This interaction is also referred to as alignment regulating force, the provision of a liquid crystal aligning function, or an alignment process.
Polyimide is often used for an alignment film of a liquid crystal display device. In a forming method for the alignment film, polyamic acid that is a polyimide precursor is dissolved in various solvents, and coated on a substrate by spin coating or printing, the substrate is heated at high temperature at a temperature of 200° C. or more, the solvents are removed, and the polyamic acid is imidized to polyimide by cyclization. The thin film has a thickness of about 100 nm in the imidization. The surface of this polyimide thin film is rubbed in a certain direction using a rubbing cloth, polyimide polymer chains on the surface are aligned in the rubbing direction, and then it is achieved that polymers on the surface are in a high anisotropic state. However, there are problems such as the occurrence of static electricity and foreign substances caused by rubbing and ununiform rubbing caused by irregularities on the surface of the substrate, and a photo-alignment method is becoming adopted in which polarized light is used to control molecular orientations with no need to contact a rubbing cloth.
The photo-alignment method for a liquid crystal alignment film includes photoisomerization type photo-alignment that the geometry in a molecule is changed by applying a polarized ultraviolet ray like azo dye and photodimerization type photo-alignment that molecular frameworks generate a chemical bond caused by a polarized ultraviolet ray such as cinnamic acid, coumalin, and chalcone, and other types. Photodecomposition type photo-alignment is suited to the photo-alignment of polyimide that is reliable and achieves results as a liquid crystal alignment film, in which a polarized ultraviolet ray is applied to polymers, only polymer chains arranged in the polarization direction are broken and decomposed and molecular chains in the direction perpendicular to the polarization direction are left.
This method is studied in various liquid crystal display modes. For the IPS mode in the various modes, Japanese Patent Application Laid-Open No. 2004-206091 discloses a liquid crystal display device that decreases the occurrence of display failures caused by changes in the initial alignment direction, stabilizes liquid crystal alignment, and improves mass production, a contrast ratio, and image quality. In Japanese Patent Application Laid-Open No. 2004-206091, the function of controlling molecular orientations is provided by performing an alignment process in which at least one secondary treatment of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure is applied to polyimide or polyamic acid formed of aromatic diamine, cyclobutanetetracarboxylic dianhydride, and a derivative of cyclobutanetetracarboxylic dianhydride, polyamic acid formed of aromatic diamine and cyclobutanetetracarboxylic dianhydride, or polyamic acid formed of aromatic diamine and a derivative of cyclobutanetetracarboxylic dianhydride. It is noted that a configuration in which the alignment film is formed in a two-layer structure is described in Japanese Patent Application Laid-Open No. 2010-72011.
More specifically, Japanese Patent Application Laid-Open No. 2004-206091 describes that the effect is further effectively exerted when at least one process of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure is performed in a temporal overlap of a polarized light irradiation process, and that the effect is also effectively exerted when an alignment control film is subjected to an imidization baking process and the polarized light irradiation process in a temporal overlap. More specifically, Japanese Patent Application Laid-Open No. 2004-206091 describes that in the case where a liquid crystal alignment film is subjected to at least one process of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure in addition to polarized light irradiation, the temperature of the alignment control film is desirably in a range of a temperature of 100 to 400° C., and more desirably in a range of a temperature of 150 to 300° C. The processes of heating, infrared irradiation, and far infrared irradiation can be combined with the imidization baking process of the alignment control film, which is effective.
However, the liquid crystal display device using these photo-alignment films has a short history as compared with the case of using rubbed alignment films, and sufficient findings are not available for long-term display quality over several years as a practical liquid crystal display device. In other words, the fact is that the relationship between image quality failures and problems unique to the photo-alignment film, which are not obvious in the initial stage of manufacture, are rarely reported.